2005 Ford F-250 side mirror defrosters don't work?!

I just bought a 2005 Ford F-250 king ranch. And since, for some reason there's no rear window defrosters. I really need the side mirror defrosters. It has the little "de frost" symbol on both mirrors. But this morning I had the defrosters on and the mirrors never defrosted. At all! Not even a little! What's wrong? I read since there's no switch to turn them on, it automatically comes on when using the defroster. I thought maybe a fuse is blown. But what fuse would it be? I looked online at the fuse diagram. And don't see one for the side mirror defrosters. Help!

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